In this period three compounds were evaluated for in vitro and in vivo stability: Iodohexestrol (IHex), 17 alpha iodoethynylestradiol (IEE2) and 11beta-methoxy 17 alpha iodoethynylestradiol (11beta OMe IEE2). Procedures have been developed to label all three compounds with I-125 at maximum specific activity of 2200 Ci/mmol using Chloramine-T. All three compounds are stable when stored at 50% ethanol-phosphate buffer. Iodohexestrol is stable in rat plasma and in the presence of uterine cytosol. IEE2 and 11 beta OMe IEE2 deiodinate rapidly in rat plasma and in the presence of uterine cytosol so that radioiodide is the only radioactive species present 2 hr after mixing. Radiolabeled IEE2 binds to rat uterine cytosol but cannot be displaced by 100 times the concentration of estrogen. Sucrose gradient analysis indicates that the binding is not to estrogen receptors. Iodohexestrol is approximately 15% bound to rat uterine cytosol. The 85% nonestrogen receptor binding is displaceable by thyroxine and ANS. This compound may be useful in in vitro estrogen receptor assays when combined with ANS.